Accurate measurement of wastewater flows in sewers and other open channels is increasingly important for both economic and environmental reasons. Existing flowmeters for open channels are of two types: head-type meters which measure water depth and are used with weirs and flumes; and velocity-area (VxA) meters which obtain flow cross section area from a measurement of depth and multiply this area by a factor based on a measurement of water velocity. Properly installed head-type meters are 1-2% accurate; VxA meters are typically at least an order of magnitude less accurate, but can be used where installation of a flume or weir is impractical or impossible.
The common weakness of VxA meters is in the measurement of water velocity. Techniques commonly used for wastewater velocity measurement are contrapropagating and reflective ultrasonics (both based on frequency shifting), electromagnetic probes, and (for short-term measurements) paddlewheels and turbines. Such devices are typically rated for velocity ranges to 30 fps (feet per second) and higher; accuracy is severely degraded at low velocities. The velocities found in open channel wastewater flows range from typical highs of 3 to 5 fps in free flow down to zero or even reverse flow under stoppage conditions.
In open channel flow applications, present velocity-measuring instruments are at best operating in the bottom 10-15% of their full range, with resulting poor accuracy. Electromagnetic probes are especially prone to fouling, with resultant calibration drift, in slowly moving flows. In wastewater flows, paddlewheel, turbine, target and other intrusive sensors typically fail in short times due to fouling and trash accumulation. There is a great need for an open channel flowmeter for wastewater which can operate accurately in pipes and channels (without using a weir or flume) which is not subject to the shortcomings of velocity measurements.
Several manual techniques are in common use for measuring water flowrate in channels without measuring velocity. These methods measure the movement of the water along the channel over a measured distance in a measured time, as contrasted with measuring the instantaneous water velocity at the location of the velocity sensor. The simplest of these techniques is the method of floats; small floats are dropped into the flow and visual measurements are taken of the travel time to a known point downstream. The water flowrate is computed as: ##EQU1## It is possible to compute a value for "average" velocity, dividing the travel distance by the travel time. The result is a fictitious value, which for most situations will not be the actual velocity of any significant portion of the flow. Regardless, this "average velocity" is a derived value; it is not a measurement, and it not required for the calculation of flowrate.
The same principle underlies dye and salt techniques. Dye may be dumped into moving water, and the time of travel observed visually. Salt may be dumped into the water, and the time of arrival at the downstream point observed with a conductivity meter. These techniques are characterized by low resolution in distance and time measurements, requiring the measurements to be made over large distances and times; rapid changes of flowrate are averaged out rather than measured. Also, for each measurement, material must be added to "tag" the water. On the positive side, the lack of velocity measurement means these techniques work equally well, though inaccurately, in both fast and slow flows. To date, this benefit has not been realized in an automated instrument because of these problems of the long baseline and need to continuously add "tagging" material.
The present invention solves these problems by using the solids already being carried by the flowing water, and by providing a precise, short-baseline method for accurately measuring the travel distances of such solids. The invention successively identifies and measures the location of individual solids at short time intervals and calculates the liquid flowrate based on the movement of these solids. Because solids are carried throughout the flow, the technique measures throughout the cross section of flow; accuracy is not dependent on laboratory-quality flow conditions. The present invention is also useful in flows without measurable solids by introducing bubbles in the flow.
Accordingly, the first object of the invention is to implement a channel flowmeter to continuously measure flowrate by measuring water movement instead of velocity.
Another object of the invention is to measure the water movement using short baselines and short times, for installation convenience and to better track short-term variations in flowrate.
Another object of the invention is to perform such non-velocity flow measurements without the addition of tagging material to the flowing water.
Another object of the invention is to take measurements throughout the flow, preserving accuracy where flow conditions are irregular.
Another object of this invention is to provide a volumetric flowmeter which accurately computes a true flow volume regardless of flow velocity, does not require a known flow profile, and is essentially independent of upstream and downstream flow conditions, including flow stoppages.
Another object of this invention to provide a volumetric flowmeter which can be used in the same applications as velocity-area meters, but which avoids the shortcomings of velocity measurement.
Another object of this invention is to provide an accurate method of measuring flowrates of object-bearing liquids moving slowly.